CD3 (cluster of differentiation 3) was discovered concurrently with the monoclonal antibody OKT3. Initially, OKT3 was found to bind to all mature, peripheral T cells, and later the CD3 epsilon subunit as part of the TCR-CD3 complex was determined to be the cell surface antigen bound by OKT3. (“Monoclonal antibodies defining distinctive human T cell surface antigens” Kung et al. 1979) OKT3 was subsequently tested as an immunosuppressant for transplant rejection with the initial trial studying acute kidney allograft rejection (“Treatment of acute renal allograft rejection with OKT3 monoclonal antibody” Cosimi et al. 1981).